An image reading apparatus reads an image on a sheet by using an image sensor such as a reduction optical system (a charge coupled device (CCD)) or a unit-magnification optical system (a contact image sensor (CIS)). The image reading apparatus may have individual variations in illumination from a light source for illuminating a reading surface of a sheet or vignetting from a lens of the image sensor. The differences occur during manufacture and also occur due to aging degradation of the apparatus. The individual variations are a factor for generating irregularities in image data by reading. Therefore, a shading correction for correcting the irregularities in the image is performed.
The shading correction requires white reference data indicating a reference value of brightness corresponding to white. The white reference data is generated by reading a white reference surface which is provided on the image reading apparatus. Therefore, if a contaminant is on the reference plate surface, suitable white reference data is not acquired and there is a possibility that accuracy in the shading correction is lowered.